Well cementing is a process used in penetrating subterranean zones (also known as subterranean formations) to recover subterranean resources such as gas, oil, minerals, and water. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement and water is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, e.g., completion and work over operations, may also be performed.
In some situations, the cement slurry is subjected to relatively high hydrostatic pressures in the well bore. As such, the density of the cement slurry needs to be sufficiently high to ensure that the cement can contain such high pressures. However, the density of conventional cement slurries containing Portland cement and water is often less than desired. One method developed for overcoming this limitation of conventional cement slurries is to increase the slurry density by adding a weighting material to the slurry. A weighting material is a material having a specific gravity higher than that of the other components in the slurry such as the Portland cement, which typically has a specific gravity in the range of 3.15 to 3.2. Commonly employed weighting materials are iron oxide (i.e., hematite) and manganese oxide (i.e., hausmannite). Hematite and hausmannite are commercially available from Halliburton Energy Services, Inc. under the trade names HI-DENSE® NO. 4 and MICROMAX, respectively, and they have an average specific gravity of 5.2 and 4.9, respectively.
In some situations, the weighting material is soluble in and thus can have an adverse effect on certain fluids in the well bore. For example, the high concentrations of soluble metal salts, such as those containing iron and manganese, in hematite and hausmannite, respectively, can contaminate certain types of brines used in completion and work over cementing operations. The presence of some weighting materials in a cement slurry also can undesirably cause the slurry to have a relatively high viscosity. Thus, the ability to pump the cement slurry into the well bore may be compromised by the use of the weighting material. A need therefore exists to develop a process for increasing the density of a cement slurry without risking contamination of fluids in the well bore and without compromising the ability of the slurry to be pumped. The present invention utilizes a new weighting material that does not adversely affect fluids in the well bore to form a pumpable cement slurry having a relatively high density.